Cu (copper) or a Cu alloy, which has high electrical conductivity and has excellent ductility and suitable strength, is widely used as a base material for terminals used for electrical connection, for example, between electrical cables. However, Cu poses the problem of high contact resistance between terminals because of the insulating film such as an oxide film or a sulfide film formed on the surface in the service environment.
To address this problem, some terminals are provided with a Sn (tin) plated film formed by performing a plating process on the surface of the base material. Sn is softer than other metals and therefore the insulating film formed on the surface of the Sn plated film can be easily broken, for example, by sliding movement between the terminals, so that the metal Sn can be exposed. As a result, terminals having a Sn plated film on the surface can easily establish a good electrical contact.
Furthermore, to reduce the coefficient of friction for sliding movement between terminals while ensuring the contact resistance reducing effect by the Sn plated film, there is disclosed a technique in which a Cu—Sn alloy covering layer and a Sn layer are formed in order on the surface of a base material made of a Cu alloy sheet (Patent Document 1). By exposing both the Cu—Sn alloy and the Sn to the surface, the coefficient of friction can be reduced while maintaining a low contact resistance because Cu—Sn alloys are harder than pure Sn, and consequently the insertion force necessary for insertion of the terminal can be reduced.